Showing papers in "Powder Metallurgy in 1998"

Compaction Equation and Its Use to Describe Powder Consolidation Behavior

Abstract: A new phenomenological equation for the compaction curve is given. This equation is easy to use, permits a quantitative description of the compaction process, and enables the characterisation of powder consolidation behaviour by two parameters. Based on several sets of published data on the compaction of both metallic and non-metallic powders, the results of linear regression analyses show that in most cases the proposed equation gives correlation coefficients closer to unity (usually 0·99 to 0·999). PM/0763

Numerical Investigation of Mechanical Behaviour of Nanocrystalline Copper

Abstract: The effect of grain size on the overall elastic and plastic properties of nanocrystalline materials has been evaluated using the elastoplastic finite element method. The tensile deformation behaviour of nanocrystalline copper (based on a unit cell model comprising 2–100 nm sized copper crystallites and 1nm wide grain boundaries) has been analysed. Assuming that the material is elastic-perfect plastic (because of the very fine grain size), it has been shown that the yield stress increases and the elastic modulus decreases with a decrease in grain size. These results are consistent with the reported experimental observations. This analysis can be used effectively to predict the mechanical behaviour of any type of nanocrystalline material. PM/0752

Full Densification of Loosely Packed W–Cu Composite Powders

Abstract: The metal injection moulding (MIM) technique appears to have potential for mass production of small and delicately shaped W–Cu parts, primarily for microelectronics, although problems in densification, owing to the inherently poor sinterability of the W–Cu system as well as the low volume fraction of MIM debound parts, need to be overcome. Several attempts have been made to attain full densification of the loosely packed W–Cu composite powders in MIM brown parts. The use of the nanocomposite W–Cu powders, prepared either by coreduction of fine Wand Cu oxide powder or by mechanical alloying of Wand Cu powders, was shown to enhance the sinterability, resulting in the full densification of W–Cu MIM parts. Similar effects were observed in the case of tailored W–Cu powder mixtures with high mixing homogeneity and packing density. The high sinterability of the pertinent W–Cu systems is attributed partly to the nanosintering effect, owing to the intrinsic homogeneous and fine mixed state of the W–Cu phas...

Effects of Processing Parameters, Particle Characteristics, and Metallic Coatings on Properties of Al2O3 Copper Alloy Matrix Composites

Abstract: The powder metallurgy technique was adopted to prepare Al2O3 copper alloy matrix composites. The mechanical properties of the composites were measured to investigate the influence of processing parameters, Al2O3 particle characteristics, and metallic coating (copper or nickel) of the Al2O3 particles. It was found that the mechanical properties of the composites prepared by pressing, sintering, re-pressing, and resintering were superior to those of composites prepared by the pressing and sintering route, and that sintering temperature was an important factor in controlling composite properties. Changes in hardness, theoretical density, ultimate tensile strength, and wear resistance of composites containing Al2O3 of varying particle size and alumina content were observed. In all cases, composites made from nickel coated or copper coated Al2O3 exhibited better properties than those from uncoated particles. PM/0759

Study of Processing Parameters for MIM Feedstock Based on Composite PEG-PMMA Binder

Abstract: Binders composed of polyethylene glycols (PEGs), polymethylmethacrylate (PMMA) and, in some cases, stearic acid have been used to injection mould 316L stainless steel powder having a median particle size of about 25 μm. The effects of changing the molecular weights of the PEGs, of changing the proportions of the PEGs and PMMA in the binders, and of additions of stearic acid on moulding behaviour and on removal of the PEGs from mouldings by water leaching have been investigated. Reducing the PMMA content allows higher solids contents to be moulded and, for a given solids content, lower moulding temperatures to be used and more rapid removal of the PEGs by water leaching. However, reducing the PMMA content lowers the stiffness and strength of the mouldings. Reducing the average molecular weight of the PEGs accelerates the leaching. Introduction of stearic acid allows higher solids contents to be successfully moulded. PM/0796

Severe Plastic Deformation: New Technique for Powder Consolidation and Grain Size Refinement

Abstract: Severe plastic deformation (SPD) processing is widely used to control and refine the grain size of monolithic metals and alloys, with grain sizes of less than 100 nm produced in recent years. One p...

Influence of sintering temperature on pore morphology, microstructure, and fatigue behaviour of MoNiCu alloyed sintered steel

Abstract: The influence of sintering temperature on the pore morphology, microstructure, and fatigue behaviour of a MoNiCu alloy is presented. Besides pore area, shape factors for bulkiness, elongation, and roughness of the pores were determined. Energy dispersive spectroscopy X-ray analyses were used to examine the distribution of alloying elements in the bulk of the material. Cyclic stress-strain, cyclic stress-temperature, and cyclic stress-electrical resistance responses were studied simultaneously to obtain a more comprehensive description of the fatigue behaviour. With increasing sintering temperature the pores are larger and more regular and, owing to a more homogeneous distribution of alloying elements, the solid solution hardening is more pronounced. Nevertheless, there is only a slight increase in the endurance limit. The higher sintering temperature leads to lower plastic strain amplitudes, temperature, and electrical resistance changes. This corresponds to less pronounced slip features at a low...

Mechanochemical Synthesis of Nanocrystalline Tungsten Carbide Powders

Abstract: Nanocrystalline tungsten carbide powder mixtures of WC, W2 C, and cubic WC1–z were prepared by the mechanochemical method of reactive high energy ball milling WO3–Mg mixtures containing graphite as...

Guide to Injection Moulding of Ceramics and Hardmetals: Special Consideration of Fine Powder

Abstract: The small particle size of ceramic and hardmetal powders significantly increases the risk of particle agglomeration with an associated increase in viscosity of plastisols used in powder injection moulding. The agglomeration is counteracted by adding surface active compounds to the powder containing plastisol. A model is presented for the functioning of such additives in injection moulding or fine (ceramic, hardmetal) powder. The idea is to create a thin, dense adsorbed layer which reduces the attractive van der Waals forces without adding considerably to particle size, thus decreasing viscosity without creating perfect stability. In this manner, the pseudoplasticity necessary for the injection moulding is retained. The approach outlined shows that the adsorbates working as dispersants should be short molecules such as stearic acid, stearyl alcohol, and octadecyl silanes, all built around a C18 chain. PM/0786

Production of Diamond Impregnated Cutting Tools

Abstract: Diamond impregnated tools consist of randomly dispersed, randomly oriented diamond crystals embedded in a metal matrix or bond. The major production route for all metal bonded diamond tools involves powder processing: mixing of the diamond and metal powders followed by cold pressing then hot pressing or sintering. An overview is given of the important process variables and their effect on tool performance, and the main industrial applications are briefly described.

Resistance of hardmetals to fracture

Abstract: Strength of WC and TiC base hardmetals and their durability in metalforming (blanking) operations have been investigated. It has been shown that transverse rupture strength R TZ, critical stress intensity factor K Ic, and ultimate plastic strain ep as individual properties do not enable exact estimation of the resistance of hardmetals to fracture (edge chipping) in blanking operations, which are characterised by complex stress states and pronounced cyclical loads of an impact nature. Instead, a new characteristic, sensitive to structure as well as porosity (internal flaws), is suggested as a reliable measure of the resistance of hardmetals to fracture: applied toughness (characteristic of toughness and reliability in application) is defined as the product of transverse rupture strength and ultimate plastic strain in compression. PM/0764

Finite Element Simulation and Experimental Validation for Multilevel Powder Compact

Abstract: The simulation of a complete cold compaction cycle for a green powder component and an experimental comparison are presented. In the modelling development, the powder was assumed to be a rate independent elastoplastic material. The process was described by a small displacement based finite element formulation and an elliptical cap yield surface was used to represent the powder densification. A friction model based on a plasticity analogy was found to be necessary in the analysis of the part ejection process. Laboratory and factory based experiments were carried out to provide detailed information about the material parameters, tooling motion, and validation of the numerical simulation work. Two examples were studied, a plain bush component and an axisymmetric flanged component. The numerical simulation results were validated against the experimental data and the comparison showed good agreement. The versatility of the simulation allows the complete representation of the green compact generation cy...

Microstructure and Mechanical Behaviour of Single Pressed and Vacuum Sintered Gas and Water Atomised 316L Stainless Steel Powders

Abstract: The present study concerns the microstructure and mechanical behaviour of 316L PM steels produced from gas and water atomised powders by single pressing and vacuum sintering. The gas atomized powder was agglomerated with an organic binder to allow pressing. After sintering at 1250°C, the densification of the two materials is similar. Increasing the sintering temperature to 1350°C results in a pronounced densification of the gas atomized powder caused by the formation of ferrite, resulting in improved mechanical properties. Sintering the water atomised powder at 1350°C leads to limited densification and thereby less improvement of the mechanical properties. Prior particle boundaries with their oxides have a stiffening effect on the materials in the water atomised conditions with relatively sharp elastic/plastic transition. The gas atomised powder materials instead have a more diffuse yielding behaviour. The effective strain hardening, normalised by the relative density, was found to be virtually in...

Silicide Phase Formation and Its Influence on Liquid Phase Sintering in 316L Stainless Steel with Elemental Silicon Additions

Abstract: Elemental additions of silicon to sintered 316L stainless steels were found to cause the formation of liquid phases at three temperatures. The first two of these, at 1060 and 1155°C, were owing to heat generated by an exothermic reaction between the steel and its silicon addition leading to the formation of a complex mixture of silicide phases involving iron, chromium, and nickel. Alloying between the silicon and 316L stainless steel then lowered the solidus temperature of the steel and a further liquid phase was formed when sintering temperatures exceeded the new solidus of 1190°C. Full sintered densities were not achieved despite the presence of liquid phase and the use of relatively high sintering temperatures. This was because of the large pores that remained at the sites where the exothermic reaction between the silicon and the stainless steel had resulted in the formation of liquid phase which subsequently migrated away into the surrounding steel by capillary flow. These large pores were no...

Production of High Strength Al—Mg—Sc Alloys by PM

Abstract: Powders of Al–Sc and Al–Mg–Sc alloys with up to 2 wt-%Sc content were produced by high energy inert gas atomisation. Microstructure and properties of the powders were compared and discussed. The po...

Characterisation of WC-12Co thermal spray powders and HPHVOF wear resistant coatings

Abstract: Fourteen commonly used, commercially available, WC–12Co thermal spray powders were characterized in terms of their particle size distribution, surface morphology, cross-sectional morphology, and phase composition. Based on the results, four powders were selected for the deposition of thermal spray coatings using the JP 5000 high pressure high velocity oxyfuel (HPHVOF) system. Dry sand rubber wheel abrasion tests were performed on the coatings in order to determine the effect of powder manufacturing method on the wear rates. The coating produced using the cast and crushed powder did not deposit well and wore through very rapidly. The abrasion tests on the remaining coatings showed that the other two powder manufacturing routes are essentially equivalent in terms of the resultant coating wear resistance. PM/0777

Properties of Anisotropic Nd(Fe, Co)B Type Sintered Magnets Produced by Powder Injection Moulding

Abstract: Anisotropic Nd(Fe,Co)B type sintered permanent magnets were fabricated by powder injection moulding (PIM) using paraffin wax as a binder. A conventional process was also used in order to compare the resultant properties with those of sintered magnets produced by PIM. Magnetic properties, microstructure, and constituents were investigated by dc fluxmeter, scanning electron microscopy, X-ray diffractometry, wavelength dispersive X-ray analysis, and ir absorption analysis. Effects of particle alignment of the sintered magnets on the magnetic properties and anisometric linear shrinkage ratios were studied. Particle alignment of sintered magnets produced by PIM was 4% lower than that of the conventional process. The PIM sintered magnet exhibited a maximum energy product of 232 kJ m−3. The residual carbon after debinding affected the magnetic properties and sintering characteristics by contaminating the liquid phase during PIM sintering. PM/0782

Magnetic Properties of Nd—Fe—B Magnets Sintered Using Granulated Powders by Spray Drying Method

Abstract: In recent years the demand for smaller and thinner Nd-Fe-B permanent magnets has increased significantly. The poor flowability of the alloy powder when feeding and compacting has however been an obstacle to their fabrication. In order to improve the powder characteristics, Nd-Fe-B alloy powder was granulated by a spray drying method using a polyvinylalcohol (PVA) binder (0·1−0·5 wt-%). The granulated powder exhibited higher flowability with increased PYA content. The green bodies compacted under a magnetic field of 0·88 MA m−1 were debindered by heating in H2 and then sintered in a vacuum. The magnetic properties of a sintered anisotropic magnet prepared by adding 0·20 wt-% PVA are close to those btained if the spray drying process is not used. The conditions required for the spray drying of Nd-Fe-B alloy powder have been investigated. PM/0779

Model for Thermal Analysis in Cold Uniaxial Powder Compaction

Abstract: A combined mechanical and thermal analysis of cold uniaxial powder compaction is presented where the analysis was coupled via friction and plastic work heating and the local density field. Experimental work confirmed that powder thermal conductivity increases rapidly with densification and that it is determined principally by the conductivity of the stock material. The numerical modelling work showed the dominant effect of frictional heat input with regard to temperature rise. The analysis also showed that compact temperature was also increased at higher compaction speed and that the effect of the interface heat transfer between the powder and tooling is important. PM/0778

Influence of Density and Sintering Temperature on Monotonic, Cyclic, and Low Cycle Fatigue Behaviour of FL4405 High Strength PM Steel

Abstract: Density and sintering temperature effects were investigated with FL4405 low alloy PM high strength steel. Composition, microstructure, image analysis, monotonic and cyclic stress-strain behaviour, low cycle fatigue, and macro- and microfractography were evaluated. Density levels of 7·0 and 7·4 g cm−3 and sintering temperatures of 1120 and 1315°C were investigated. The increased density resulted in smaller pore size and pore volume while the increased sintering temperature resulted in greater pore roundness. All final fracture surfaces were brittle at the macroscale while exhibiting ductile dimpling at the microscale. Low cycle fatigue surfaces contained principally ductile dimples with no striations or benchmarks. Increasing the density resulted in significantly higher tensile strength, elastic modulus, ductility, and low cycle fatigue resistance. Increasing the sintering temperature, and hence increasing pore roundness, resulted in additional enhancement of these properties. However, this increa...

Influence of density and sintering temperature on fatigue crack growth and fracture toughness of FL4405 high strength PM steel

Abstract: Density and sintering temperature effects were investigated with FL4405 low alloy PM high strength steel. Room temperature, plane strain fracture toughness K Ic, and constant amplitude fatigue crack growth with (min/max) load ratios of 0·05 and 0·5 and macroand microfractography were evaluated. Two density levels, 7·0 and 7·4 g cm−3, and two sintering temperatures, 1120 and 1315°C, were investigated. Increasing the density and sintering temperature had no influence on stress intensity threshold and near threshold fatigue crack growth resistance where transcrystalline/transparticle cleavage existed. At higher fatigue crack growth rates, where ductile dimples existed, both higher density and higher sintering temperature enhanced fatigue crack growth resistance. The K Ic and K max values at fatigue crack growth fracture increased at the higher density and increased further at the higher sintering temperature. All final fracture regions were brittle at the macroscale while containing ductile dimples ...

Segregants at Prior Particle Boundaries in Powder Metallurgical Martensitic Stainless Steel

Abstract: The segregant behaviour in powder metallurgical (PM) martensitic stainless steel produced by hot isostatic pressing of gas atomised powder has been investigated by characterisation of the surface products on atomised powder and of the precipitate pattern on the prior particle boundaries (PPBs) in the consolidated material. Special attention was paid to the distribution of minor alloy constituents and impurities. Attempts were made to correlate the PPBs with microstructural features such as former austenite grain boundaries (FAGBs). The powder surfaces were analysed using electron spectroscopy for chemical analysis, Auger electron spectroscopy, and secondary ion mass spectroscopy (SIMS). The particles were covered primarily by Fe, Cr, and Mn oxides. Compared with the alloy composition Mn and Cr were preferentially oxidised. Oxides of the minor constituents W, Nb, and V were also found in the surface layer. In B alloyed material this element was enriched within the surface oxide. The impurities Sn,...

Stage 1 compaction behaviour of tool steel powder under die pressing

Abstract: The stage 1 compaction behaviour of tool steel powder under die pressing was studied. The effects of friction between the powder and the die wall were also investigated under different die pressing modes. The e1astop1asticconstitutive equations based on the yield functions by Fleck et al. and by Shima and Oyane were implemented in a finite element programme to simulate die compaction processes. Finite element calculations were compared with experimental data for densification and density distributions of tool steel powder under single and double action die pressing. Finite element calculations using the yield function by Fleck et al. agreed better with experimental data than those by Shima and Oyane. PM/0775

Influence of density and sintering temperature on smooth, notched, and cracked variable amplitude fatigue behaviour of FL4405 high strength PM steel

Abstract: Density and sintering temperature effects were investigated with FL4405 low alloy PM high strength steel. Room temperature variable amplitude loading was applied to smooth axial specimens, keyhole notch specimens, and compact disc precracked specimens. Each block of the variable amplitude load spectrum contained 20 127 reversals involving both tensile and compressive mean loads. Two density levels, 7·0 and 7·4 g cm−3, and two sintering temperatures, 1120 and 1315°C, were investigated. Increasing the density decreased pore size and volume while increasing the sintering temperature increased pore roundness. All final fracture regions were brittle at the macroscale and contained ductile dimples at the microscale. Fatigue regions contained principally cleavage, principally ductile dimples, or mixtures of cleavage and ductile dimples. Keyhole notch specimens with K t = 3·5 were fully notch sensitive for all material conditions. Increasing the density resulted in a significant increase in fatigue resist...

Metal Injection Moulding Powder Produced by High Pressure Water Atomisation

Abstract: Recent progress in metal injection moulding (MIM) technology has been remarkable and, as a characteristic parts manufacturing technology, it has succeeded in achieving a certain position. However, improvement of raw material powder quality and reduction of powder price are indispensable for further advances. To achieve these targets, Pacific Metals Co. Ltd is promoting the development of MIM powder production using high pressure water atomisation and has succeeded in mass production of suitable powders. These newly developed MIM powders consist of water atomised powders with low oxygen content and high tap density. In addition to stainless steel, which is the mainstream conventional MIM powder, low alloy steel powder for utilisation in the manufacture of automobile and machine parts has been successfully produced. The production technology used to make these MIM powders is described. PM/0790

Diamond Grinding and PCD Machining of MMC Engineering Components

Abstract: Many of the problems associated with the machining of metal matrix composites (caused by the presence of the hard, brittle phase) can be overcome by using ultrahard tooling. Tools made from polycrystalline diamond (PCD) are particularly suitable, however it is important that an appropriate PCD grade and optimised machining conditions are selected if rapid tool wear is to be avoided. This overview concludes with a number of case studies demonstrating how ultrahard tooling technology has been economically applied in a diverse range of engineering sectors.

Microstructural Development in Rapidly Solidified Powder of M2 Type High Speed Steel During Annealing

Abstract: Rapidly solidified (RS) powder of M2 type high speed steel (HSS) prepared by nitrogen spraying was annealed in the temperature range 150–1200°C A minor decrease in microhardness of the powder particles was determined after annealing below 400°C Beyond this temperature, the microhardness increased, principally because of the hardening effect of carbide precipitates in the material The RS powder material became overtempered above a temperature of approximately 550°C, which was determined as the maximum precipitation hardening temperature and the structure typical for soft annealed HSS origins in powder particles The main structural constituents were the bcc phase, M6C, and MC carbides No other carbides were found Overheating the RS powder above the critical temperature A 1 resulted in almost complete austenitising of the material, partial dissolving of M6C carbides, and complete breakdown of the as solidified microstructure PM/0751

Effect of Tempering Temperature on Sintered Properties of Sinter Hardened PM Steels

Abstract: The development of low alloy steel powders specifically designed for sinter hardening applications enables components to reach high sintered strength and apparent hardness in sintering furnaces equipped with either conventional or rapid cooling units. By adjusting the mix formulation, the sintered properties can be tailored to optimise both strength and apparent hardness. As untempered martensite is a fragile structure, sinter hardened parts are generally given a tempering treatment to restore strength and improve toughness. However, this treatment also affects the apparent hardness of PM parts. A study was carried out to evaluate the effect of tempering temperature on mechanical properties of sinter hardened steels containing 2% copper and either 0.65 or 0.80% carbon. Testpieces were pressed to 6.8 g cm -3 and sintered at 1120°C in a production furnace equipped with a conventional cooling unit. Three tempering temperatures, 150, 175, and 205°C, were selected for the test programme. A reduction in apparent hardness of about 10 HRC (85 HV) was observed after tempering at 150°C but tensile properties were significantly increased, particularly at high carbon concentration.

High velocity oxyfuel thermal sprayed coatings : processing, characterisation, and performance

Abstract: The Department of Materials Engineering and Materials Design at Nottingham University has developed a thermal spraying capability using the high velocity oxyfuel (HVOF)process. Over the past three years focused research has been conducted using a gas fuelled system. Recently a state of the art liquid fuelled system has been installed which will open up many new avenues for research. Particular strengths of the department are its capabilities for undertaking detailed microstructural characterisation studies of coatings using advanced X-ray and electron optical techniques, combined with expertise in evaluating coating performance through numerous types of wear and corrosion test procedures.